Method of determining effect of radio wave multipath fading

ABSTRACT

The invention relates to a method of determining the average effect of radio wave multipath fading in a desired area in a radio system. The desired area of a coverage area of a base station in the system is described by a digital map. The average effect of multipath fading of a transmitter output is determined in different sub-areas of the desired area. In order to reduce the computation amount signal fading parameters are determined at some point of each sub-area by a ray tracing method and the average effect of signal multipath fading in the sub-area is calculated using said parameters.

FIELD OF THE INVENTION

The invention relates to a method of determining the average effect ofradio wave multipath fading in a desired area in a radio system, saidmethod using an at least two-dimensional digital map to describe thedesired area of a covering area of a base station in the system, andsaid method determining the average effect of multipath fading of atransmitter output in different sub-areas of the desired area.

BACKGROUND OF THE INVENTION

When a radio system is being constructed an attempt is made to achieve adesired coverage area with minimum costs. When considering the locationsof the base stations of the system, the required traffic capacity andthe achieved coverage area are taken into account. The aim is to locatethe base stations so as to ensure an extensive coverage area and anadvantageous location of the base station as far as the radio wavepropagation is concerned.

Different methods and instruments have been developed for radio networkplanning. Digital maps providing modelled information on the terrain andbuildings in the desired area are commonly used instruments in radionetwork planning. By means of a digital map a computer can be used tocalculate coverage areas and parameters concerning the network operationfor different base station locations.

When determining the base station coverage area it is important todetermine signal fading in different areas. As the system behaviour issimulated signal fading also has to be modelled as realistically aspossible. Particularly in an urban environment fading is a variablechanging in time depending on the geometry of the buildings, walls andother scattering surfaces surrounding the base station and the terminal.

Fading is conventionally divided into two different types, fast and slowfading, but this is a very rough division. In reality signal multipathfading is caused by phase differences, and as the terminal moves thephase difference change results in back and forth variation of thesignal strength typically seen at correlation distances from half awavelength to hundreds of wavelengths depending on the environment.

In prior art solutions, in moving radio system simulators, for examplein link-level simulators implemented by COSSAP, fadings are simulated byadding attenuation and fading to the transmitted signal. Fast fading istypically generated by simulating a stationary random process accordingto Rayleigh or Rice distributions. Hereafter fading is averaged using anappropriate filter.

In network planning software using the digital map, the signalattenuation is calculated, for example, on the basis of an empiricalmodel where multipath fading is not taken into account except forcertain special cases.

However, prior art methods have several problems and defaults. Intypical link-level simulators implemented, for example by COSSAP, thereceiving algorithms are realistic, but the problem is whether thechannel model is realistic or not. A realistic channel model can beattained by ray tracing i.e. a ray search channel model. This is,however, obstructed by the computational complexity required by a raytracing method impeding the implementation of the method.

In network planning software the resolution of the digital map can beraised to increase accuracy but then fading should be taken into accountin order to calculate the strength of the field by the map resolution.In this case too, the ray tracing model taking multipath fading intoaccount would improve accuracy but the required computation amount istoo high.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a method by which theaverage effect of fading can be calculated with acceptable accuracywithout a high computation load.

This is achieved by the method of the type set forth in the preamblecharacterized in that signal fading parameters are determined at somepoint in each sub-area by a ray tracing method and that the averageeffect of signal multipath fading in a sub-area is calculated using saidparameters.

The method of the invention has several advantages. In the solution ofthe invention the computation time is short, a fraction of the timetaken if the entire calculation were performed using the ray tracingmethod, in which case calculating an average for the sub-areas wouldrequire several computation points for each sub-area. The ray tracingmethod is currently the most accurate known method for obtainingrealistic results of fading and attenuation values. In the solution ofthe invention, when the size of the sub-areas is appropriately selected,an estimation result that is acceptably accurate is, however, obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail withreference to the examples of the accompanying drawings, in which

FIG. 1 illustrates a radio system, to the planning of which the methodof the invention can be applied,

FIG. 2 illustrates parameters that can be obtained by a ray tracingmethod,

FIG. 3 illustrates an area by which fading is estimated and

FIGS. 4a and 4 b illustrate computational results.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention can thus be preferably applied in radiosystem planning. It is particularly suitable for planning radio systemsimplemented in an urban environment. FIG. 1 illustrates some typicalradio systems. The Figure shows a cell 100 of a typical cellular radionetwork. A base station 102 of the cellular radio network is locatedoutside the buildings comprising a plurality of subscriber terminals104-108, some 104, 106 of which can be located outdoors and some 108inside buildings 110. The subscriber terminals have a bidirectionalconnection to the base station.

The solution of the invention is based on the fact that multipath fadingis caused by varying phase differences of multipath-propagated signalcomponents arriving at a receiver from a transmitter. The aim istherefore to determine the average fading strength of a transmitteroutput in different sub-areas of a desired area. The sub-areas can, forexample, be squares, the size of which being in accordance with theresolution of the digital map.

In the solution of the invention signal fading parameters are determinedat one point in some sub-area of the desired area by a ray tracingmethod. In said point the amplitude, phase shift and arrival angle ofthe signal components are calculated. Next, it is assumed that theattenuation and arrival angle of the signal are constants in the entiresub-area and the average signal strength is determined by integratingover the sub-area using said parameters. The assumption about theparameters being constants is valid when the dimensions of the area tobe examined are small compared with the ones of the surroundingscattering surfaces, such as wall dimensions.

Let us take a closer look at an example, and assume that the ray tracingmethod is used to determine the amplitude a_(i), phase φ_(i) and arrivalangle _(i)α of rays i that have arrived at a point. The averageeffective signal strength by a line segment can be calculated accordingto the following formula. $\begin{matrix}{P_{ave} = {\frac{1}{b}{\int_{{- b}/2}^{b/2}\quad {{{\sum\limits_{m = 1}^{M}{a_{m}^{{{- j}\quad \phi_{m}} + {j\quad {kx}\quad \cos \quad a_{m}}}}}}^{2}{x}}}}} \\{= {{\sum\limits_{m = 1}^{M}{a_{m}}^{2}} + {\frac{4}{kb}{\overset{M}{\sum\limits_{\underset{n > m}{m,{n = 1}}}}{a_{m}a_{n}\cos \quad ( {\phi_{m} - \phi_{n}} )*\frac{\sin \lbrack {k\quad \frac{b}{2}( {{\cos \quad \alpha_{m}} - {\cos \quad \alpha_{n}}} )} \rbrack}{{\cos \quad \alpha_{m}} - {\cos \quad \alpha_{n}}}}}}}}\end{matrix}$

The average effective signal strength for a square pixel can, in turn,be calculated by the formula $\begin{matrix}\begin{matrix}{P_{ave} = \quad {\frac{1}{{b1}^{2}}\quad {\int_{- \frac{b1}{2}}^{\frac{b1}{2}}{\int_{- \frac{b1}{2}}^{\frac{b1}{2}}{{{\sum\limits_{m = 1}^{M}{a_{m}^{{{- j}\quad \phi_{m}} + {j\quad {k({{x\quad \cos \quad {a1}_{m}} + {y\quad \sin \quad {\alpha 1}_{m}}}\quad)}}}}}}^{2}{x}{y}}}}}} \\{= \quad {{\sum\limits_{m = 1}^{M}{a_{m}}^{2}} + {\frac{4}{k^{2}{b1}^{2}}{\overset{M}{\sum\limits_{\underset{n > m}{m,{n = 1}}}}{a_{m}a_{n}\cos \quad ( {\phi_{m} - \phi_{n}} )*}}}}} \\{\quad \frac{{\sin \lbrack {k\quad \frac{b1}{2}( {{\cos \quad {\alpha 1}_{m}} - {\cos \quad {\alpha 1}_{n}}} )} \rbrack}{\sin \lbrack {k\quad \frac{b1}{2}\quad ( {{\sin \quad {\alpha 1}_{m}} - {\sin \quad {\alpha 1}_{n}}} )} \rbrack}}{( {{\cos \quad {\alpha 1}_{m}} - {\cos \quad {\alpha 1}_{n}}} )( {{\sin \quad {\alpha 1}_{m}} - {\sin \quad {\alpha 1}_{n}}} )}}\end{matrix} \\{where} \\\begin{matrix}M & = & {{number}\quad {of}\quad {significant}\quad {rays}\quad {at}\quad {the}\quad {observation}\quad {point}} \\a_{i} & = & {{amplitude}\quad {of}\quad {ray}\quad i} \\\phi & = & {{{phase}\quad {of}\quad {the}\quad i\text{:}{th}\quad {ray}\quad {at}\quad {the}\quad {observation}\quad {point}},} \\\alpha_{i} & = & {{angle}\quad {of}\quad {the}\quad i\text{:}{th}\quad {ray}\quad {in}\quad {relation}\quad {to}\quad {the}\quad {direction}\quad {vector}\quad {of}} \\\quad & \quad & {{{the}\quad {terminal}},} \\{\alpha 1}_{i} & = & {{{angle}\quad {of}\quad {the}\quad i\text{:}{th}\quad {ray}\quad {in}\quad {relation}\quad {to}\quad {the}\quad {pixel}\quad {side}},} \\k & = & {{wave}\quad {number}\quad ( {2\quad {\pi/\lambda}} )} \\b & = & {{averaging}\quad {length}\quad {or}\quad {instantaneous}\quad {terminal}\quad {speed}} \\\quad & \quad & {{{multiplied}\quad {by}{\quad \quad}{averaging}\quad {time}},} \\{b1} & = & {{width}\quad {of}{\quad \quad}{the}\quad {{pixel}.}}\end{matrix}\end{matrix}$

FIG. 2 illustrates the values in the case of the square pixel.

By means of the invention it is thus possible to calculate fadings innarrowband radio systems where the average signal strength or signalpower is used as a planning parameter. When the method of the inventionis used in a simulator of a moving radio network system, the ray tracingmethod needs to be used only at one point in an averaging interval. Theaveraging interval is generally short, for example 480 ms for GSM,meaning that the default on the arrival angle and attenuation remainingconstant is valid. Realistic simulations are possible since the basisfor the channel model is the correct geometry of buildings and walls.

In prior art network planning software multipath fading has generallynot been taken into account except for ground reflection. However, it isimportant to take reflections into account if great accuracy is desired.

Let us look closer at the method of the invention by means of anexample. FIG. 3 illustrates a digital map in a typical environment inwhich a cell is to be located for the radio system. The map comprisesthe structural parts of the area affecting radio wave propagation. Theaim is to locate the base station in the area so as to achieve the bestpossible audibility in different parts of the area. In practice someprobable locations where the base station could be located are selectedand a calculation is performed using these defaults. Finally the bestalternative is selected to be the final location of the base station. InFIG. 3 the base station location is marked with the letters Tx and it isassumed that the terminal travels along the path marked by the dottedline.

In this example fading is calculated both by using the method of theinvention and, for comparison, by measuring while the terminal istraveling along the marked path in the environment described in the map.FIG. 4a shows the received signal power when the sampling interval is11.1 ms and the terminal speed 5-10 km/h. Distance is shown on thehorizontal axis. A curve 400 based on measurement results is raisedupwards by 40 dB in order to enable the comparison with a simulatedcurve 402. Correspondingly FIG. 4b shows the received signal when theaveraging interval is 0.5 s. A curve 404 based on measurement results ishere too raised upwards by 40 dB to enable the comparison with asimulated curve 406. When calculating the simulation results in FIG. 4beach averaging interval includes the parameters of only one point thatare calculated by the ray tracing method. This refers to 200 ray tracingcalculations. When the number of data points exceeds 9000, thecomputation amount is significantly reduced compared to if everythingwere to be calculated by the ray tracing method. The Figure shows thatit is difficult to predict individual fading holes, but statisticallythe results are good.

The method of the invention can also be applied in broadband radiosystems. Except for radio systems where the average signal strength orsignal power is used as a system parameter, the method of the inventioncan also be applied in systems where the average power has anothermeaning.

Even though the invention has been described above with refererence tothe examples of the accompanying drawings, it is obvious that theinvention is not restricted thereto but can be modified in various wayswithin the scope of the inventive idea disclosed in the attached claims.

What is claimed is:
 1. A method of determining the average effect ofradio wave multi-path fading in a desired area in a radio system,comprising: using an at least a two-dimensional digital map to describethe desired area of a covering area of a base station in the system; anddetermining the average effect of multipath fading of a transmitteroutput in different sub-areas of the desired area, wherein determiningsignal fading values at only a single point of each sub-area by a raytracing method, setting parameters of a given empirical formula usingthe values obtained using ray tracing calculations on the respectivesingle point of each sub-area, and calculating the average effect ofsignal multipath fading in the sub-area surrounding the single pointusing the empirical formula.
 2. A method as claimed in claim 1, whereinat some point of each sub-area of the desired area the amplitude, phaseshift and arrival angle of rays arriving at said point are calculated bythe ray tracing method.
 3. A method as claimed in claim 1, wherein anaverage signal strength is determined in the different sub-areas of thedesired area using the parameters calculated by the ray tracing method.4. A method as claimed in claim 1, wherein when determining the averageeffect of signal multipath fading, one point in each averaging intervalis calculated by the ray tracing method.
 5. A method as claimed in claim1, wherein the radio system is a cellular radio system.